The present invention relates to methods for synthesis of xylenes by catalytic methylation of toluene and benzene, and more particularly relates, in one embodiment, to methods for direct, selective synthesis of para-xylene by catalytic methylation of toluene and benzene using fuel syngas.
Of the xylene isomers, i.e., ortho-, meta-, and para-xylene, the para-xylene (PX) is of particular value as a large volume chemical intermediate in a number of applications being useful in the manufacture of terephthalates which are intermediates for the manufacture of PET. One source of feedstocks for manufacturing PX is by disproportionation of toluene into xylenes. One of the disadvantages of this process is that large quantities of benzene are also produced. Another source of feedstocks used to obtain PX involves the isomerization of a feedstream that contains non-equilibrium quantities of mixed ortho- and meta-xylene isomers (OX and MX, respectively) and is lean with respect to PX content. A disadvantage of this process is that the separation of the PX from the other isomers is expensive.
Zeolites are known to catalyze the reaction of toluene with other reactants to make xylenes. Some zeolites are silicate-based materials which are comprised of a silica lattice and, optionally, alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term xe2x80x9czeolitesxe2x80x9d includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorus oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms xe2x80x9czeolitexe2x80x9d, xe2x80x9czeolitesxe2x80x9d and xe2x80x9czeolite materialsxe2x80x9d, as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, borosilicates, ferrosilicates, and the like.
The term xe2x80x9czeolite, xe2x80x9czeolitesxe2x80x9d, and xe2x80x9czeolite materialsxe2x80x9d as used herein, besides encompassing the materials discussed above, shall also include aluminophosphate-based materials. Aluminophosphate zeolites are made of alternating AlO4 and PO4 tetrahedra. Aluminophosphate-based materials have lower acidity compared to aluminosilicates. The lower acidity eliminates many side reactions, raises reactants"" utilization, and extends catalyst life. Aluminophosphate-based zeolites are often abbreviated as ALPO. Substitution of silicon for P and/or a P-Al pair produces silicoaluminophosphate zeolites, abbreviated as SAPO.
Processes have been proposed for the production of xylenes by the methylation of toluene using a zeolite catalyst. For instance, U.S. Pat. No. 3,965,207 involves the methylation of toluene with methanol using a zeolite catalyst such as a ZSM-5. U.S. Pat. No. 4,670,616 involves the production of xylenes by the methylation of toluene with methanol using a borosilicate zeolite which is bound by a binder such as alumina, silica, or alumina-silica. One of the disadvantages of such processes is that catalysts deactivate rapidly due to build up of coke and heavy by-products. Another disadvantage is that methanol selectivity to para-xylene, the desirable product, has been low, in the range of 50 to 60%. The balance is wasted on the production of coke and other undesirable compounds.
It has been further demonstrated that alkylaromatic compounds can be synthesized by reacting an aromatic compound such as toluene with a mixture of carbon monoxide (CO), carbon dioxide (CO2), and hydrogen (H2) (synthesis gas) at alkylation conditions in the presence of a catalyst system, which comprises (1) a composite of oxides of zinc, copper, chromium, and/or cadmium; and (2) an aluminosilicate material, either crystalline or amorphous, such as zeolites or clays; as disclosed in U.S. Pat. Nos. 4,487,984 and 4,665,238. Such catalyst systems, however, are not capable of producing greater than equilibrium concentrations of para-xylene (PX) in the xylene-fraction product. Typically, the xylene-fraction product contains a mixture of xylene isomers at or near the equilibrium concentration, i.e., 24% PX, 54% MX, and 22% OX. The lack of para-xylene selectivity in alkylation of toluene with syngas can be caused by (1) the acidic sites on the surface outside the zeolite channels, and/or (2) the channel structure not being able to differentiate para-xylene from its isomers. It would be desirable for the toluene alkylation to be more para-alkyl selective due to the much higher value of PX compared to that of MX and OX. Furthermore, such processes suffer from catalyst deactivation as well. In addition, the prior art disclosed neither syngas alkylation to alkyl aromatic compounds nor syngas selective alkylation to high purity PX using alumino-phosphate-based materials.
It has been recognized that certain zeolites can be modified to enhance their molecular-sieving or shape-selective capability. Such modification treatments are usually called xe2x80x9czeolite selectivation.xe2x80x9d Selectivated zeolites can more accurately differentiate molecules on the basis of molecular dimension or steric characteristics than the unselectivated precursors. For example, silanized ZSM-5 zeolites adsorbed PX much more preferentially over MX than untreated ZSM-5. It is believed that the deposition of silicon oxide onto zeolite surfaces from the silanization treatment has (1) passivated the active sites on the external surface of zeolite crystals, and (2) narrowed zeolitic pores to facilitate the passage of the smaller PX molecules and prevent the bigger MX and OX molecules from entering or exiting from the pores. In this application, the term xe2x80x9cpara-alkyl selectivationxe2x80x9d refers to modifying a catalyst or catalytic reaction system so that it preferentially forms more PX than the expected equilibrium proportions relative to the other isomers.
Zeolite selectivation can be accomplished using many techniques. Reports of using compounds of silicon, phosphorous, boron, antimony, coke and other carbon compounds, magnesium, etc. for selectivation have been documented. Unfortunately many, if not most of the zeolites used in the prior art have undesirably short active lifetimes before they deactivate and have to be reactivated or replaced.
There remains a need for still further improved processes for catalytic PX synthesis which minimizes or avoids the disadvantages of prior systems, which include low PX selectivity, rapid catalyst deactivation, and the like. Further, it would be desirable if a method for selective methylation to PX could be devised which employed a low cost source of methylating agent, such as fuel syngas.
Accordingly, it is an object of the present invention to provide a method in which PX of high product concentration is synthesized via alkylation of toluene and/or benzene with fuel syngas in a catalytic reaction system.
It is another object of the present invention to provide a method for producing PX in greater than equilibrium product concentration, e.g. greater than 30%, in the xylene product fraction as compared to prior, equilibrium concentrations of about 24%.
Still another object of the invention is to provide a method for the direct, selective production of PX from toluene and/or benzene which has a high aromatic conversion, e.g. at least above 5%, preferably above 15%, most preferably as high as possible.
In carrying out these and other objects of the invention, there is provided, in one form, a method for forming para-xylene (PX) from fuel syngas involving, optionally desulfurizing the fuel syngas, reacting the fuel syngas on a once-through basis with a feed containing an aromatic compound of toluene, benzene and mixtures thereof, in the presence of a catalytic reaction system which converts at least 5% of the aromatic compound to a mixture of xylenes, where PX comprises at least 30% of the mixture of xylenes. A methylating agent formed from the fuel syngas reacts with the PX.
It should be stressed that the invention provides a process for increased selectivity to para-xylene. Further, the invention employs aluminophosphate-based catalysts for selective para-xylene synthesis, whether or not the aluminophosphate-based materials are para-alkyl selectivated. Additionally, the invention prolongs catalyst lifetime by reducing, even eliminating catalyst deactivation, as compared with prior PX forming processes. Another advantage of the invention is the simultaneous cracking of paraffins and olefins present while processing unextracted toluene, or unextracted mixtures of toluene and benzene, as well as reacting fuel syngas on a once-through basis.